1. Field of the Invention
This invention relates to a fixing belt used in image-forming apparatus such as electrophotographic apparatus and electrostatic-recording apparatus, and an image heat fixing assembly with which unfixed images formed and held on recording mediums are subjected to heat fixing.
2. Related Background Art
In the image-forming apparatus, heat roller type assemblies have been in wide use as fixing assemblies with which unfixed images (toner images) of intended information which have been formed and held on recording mediums (such as transfer material sheets, electrofax sheets, electrostatic-recording paper, OHP sheets, printing paper and format paper) by a transfer system or a direct system at a zone where an electrophotographic process, an electrostatic-recording process or a magnetic-recording process is carried out are heat-fixed as permanently fixed images to the recording mediums. Such assemblies are commonly those making use of a heat source such as a halogen heater in the roller.
Meanwhile, as heating systems, those in which a resin belt or metal belt having a small heat capacity is heated using a ceramic heater as a heat source are widely proposed and carried out. More specifically, in such heating systems, it is common to make a heat-resistent belt (fixing belt) held between the ceramic heater as a heating element and a pressure roller as a pressing means to form a nip, and guide a recording medium on which unfixed toner images to be imagewise fixed have been formed and held, into the part between the fixing belt and the pressure roller at the nip so that the recording medium is transported together with the belt while being held between them, to impart the heat of the ceramic heater to the recording medium at the nip via the belt, where the unfixed toner images are heat-and-pressure-fixed to the recording medium surface by the heat and the pressure at the nip.
As fixing assemblies of this belt heating system, on-demand type assemblies can be set up using a low-heat-capacity member as the belt. More specifically, the ceramic heater as a heat source may be electrified only when the image-forming apparatus performs image formation, to bring the heater into a state it has generated heat at a stated fixing temperature. Thus, there is an advantage that the image-forming apparatus can have a short waiting time from power source ON to a image formation performable state (i.e., quick-start performance) and hence can enjoy a low power consumption when it is on stand-by (i.e., power saving).
As fixing belts of such a belt heating system, heat-resistant resin belts are used. In particular, polyimide resin belts are used as having good strength. However, where machines are made to have more high-speed and high-durability, such resin belts (films) are insufficient in respect of the strength. Accordingly, it is proposed to use a belt having a base layer formed of a metal having superior strength, as exemplified by SUS stainless steel, nickel, copper or aluminum.
Also proposed is, as disclosed in Japanese Patent Application Laid-open No. 7-114276, an induction heating system in which a metal belt is used and this belt is made to generate heat by itself through eddy currents produced by electromagnetic induction. More specifically, a heating assembly is proposed in which eddy currents are induced in the belt itself or in a conductive member set to be adjacent thereto, by a variation of magnetic flux to make it generate heat by Joule effect. This electromagnetic-induction system can set heat generation area closer to the member to be heated, and hence can achieve an improvement in efficiency of the energy to be consumed.
As methods of driving the fixing belt of the fixing assembly of a belt heating system, available are, e.g., a method in which a belt brought into pressure contact between a pressure roller and a belt guide which guides the inner surface of the belt is rotated by the rotational driving of the pressure roller (pressure roller drive system), and a method in which in reverse the pressure roller is rotated by the driving of an endless-belt type belt put over a drive roller and a tension roller.
As fixing belts making use of a metal belt, the use of a fixing belt made of nickel with a surface roughness of less than 0.5 xcexcm and a thickness of about 40 xcexcm is disclosed as an example in Japanese Patent Application Laid-open No. 7-13448; and in Japanese Patent Application Laid-open No. 6-222695 a fixing belt made of nickel with a thickness of from 10 to 35 xcexcm, having on its outer periphery a coating layer having releasability and on its inner periphery a resin layer.
Endless belts made of nickel are readily obtainable by a nickel electroforming process. Conventionally, the nickel electroforming process is utilized for the purpose of improving wear resistance or providing glossiness as decorative use. Hence, the resulting electroformed nickel usually contains sulfur in a large quantity. Where this electroformed nickel is used in the fixing belt, a problem may arise in durability because of embrittlement due to sulfur in a high-temperature condition.
As countermeasures therefor, as disclosed in Japanese Patent Application Laid-open No. 10-48976, a fixing belt is proposed which is comprised of a nickel metal layer containing 0.04% by weight or less of sulfur and 0.2% by weight or more of manganese, for the purpose of improving heat resistance and durability. As also disclosed in Japanese Patent No. 2706432, a fixing belt is proposed which employs as a substrate an endless electroformed sheet formed of a nickel-manganese alloy containing from 0.05 to 0.6% by weight of manganese and having a Vickers microhardness of from 450 to 650.
However, in the case of the belt heating system, in particular, the belt heating system making use of the metal belt, the belt tends to fatigue mechanically because it flexes repeatedly at the nip and in its backward and forward vicinity as the belt itself is rotated. Accordingly, it is sought to more improve the heat resistance and durability.
Now, it is considered that both strength and toughness of materials can be better achieved as the materials have a smaller crystal grain diameter. Under the conditions of electroforming, however, large-crystal texture is obtained. Hence, usually, a primary brightener containing sulfur used as a stress reducer is added. The sulfur-nickel compound formed on the cathode surface (mold) is in the form of microscopic grains, and hence has a crystal grain diameter which is smaller in about double figures. Thus, it can impart glossiness to electroformed products, but inevitably has too small a crystal grain diameter.
An object of the present invention is to provide a fixing belt having high durability and an image heat fixing assembly having high durability and high reliability, in an image heat fixing assembly which can realize low-energy heating by utilizing a heating element with a small heat capacity.
The present invention is a fixing belt having at least a release layer and a metal layer formed of electroformed nickel;
the electroformed nickel having, in its crystal texture, crystallites having an average size of from 0.05 xcexcm or more to 0.2 xcexcm or less. 
The present invention is also an image heat fixing assembly having the above fixing belt and a pair of pressure contact members which are mutually in pressure contact via the fixing belt; the inner surface of the fixing belt being slidable on one of the pressure contact members, and an image held on a recording medium being heat-fixed by the aid of the heat conducted from the fixing belt.
The present invention is still also an image heat fixing assembly having a magnetic-flux generation means which produces-a magnetic flux, and the above fixing belt generates heat in virtue of the magnetic flux produced by the magnetic-flux generation means to heat and fix the image held on a recording medium.
In the present invention, the crystallites in the crystal texture of the electroformed nickel are made to have an average size of from 0.05 xcexcm or more to 0.2 xcexcm or less. This makes it possible to provide a fixing belt having superior high durability, in particular, durability at high temperature, and an image heat fixing assembly having high durability and high reliability.